This invention relates to resin particles having specific shapes and suitable for use in the field of electronic industry such as LCD spacer, surface modifier for silver salt film, film modifier for magnetic tape, running stabilizer for heat-sensitive paper, etc.; field of rheology controlling agent, ink and adhesive; medical field such as particle for detecting antigen-antibody reaction, etc.; field of cosmetics such lubricant, etc.; or field of other general industries such as low shrinking agent, paper, dental material, modifier of resin, etc., and to preparation method thereof.
Various kinds of resin particles are used for various purposes, for example, for improving mechanical properties of plastics, for improving cleaning properties without changing the chargeability of developing agent, for improving flatting or opacifying effect of paint, for improving slip properties of cosmetics, and the like.
Since these resin particles are prepared by grinding method, emulsion polymerization method, suspension polymerization method, seed polymerization method, dispersion polymerization method, or the like, only indeterminate resin particles or spherical resin particles are usually obtained, and these resin particles are not sufficiently enough for a use in which further function is required.
For instance, flat resin particles obtained by emulsion polymerization method and having concavity are disclosed in Japanese Patent Kokai No. Hei 2(1990)-14222. However, since the resin particles are prepared by emulsion polymerization method, it is difficult to obtain flat particles having size of 1 xcexcm or more. Further, non-spherical hollow resin particles obtained by suspension polymerization method are disclosed in Japanese Patent Kokai No. Hei 3(1991)-234734, and wooden-bowl shaped fine particles obtained by suspension polymerization method and their preparation method are disclosed in Japanese Patent Kokai No. Hei 5(1993)-317688 and U.S. Pat. No. 5,559,202. And further, wooden-bowl shaped polymer particles obtained by co-polymerization of a monomer containing acid group and a monomer which can be co-polymerized with it and their preparation method are disclosed in WO98/34969 derived from Japanese Patent Kokai No. Hei 10(1998)-218950 and Japanese Patent Kokai No. Hei 10(1998)-217608.
Since these resin particles are hollow and do not have so remarkable characteristics in their shapes in comparison with spherical particles, they emit strong scattering light, and only particles of limited resin composition can be obtained. Therefore, these resin particles are not enough for exhibiting various functions required.
On the other hand, plate-like particles and disk-like particles and their preparation method are disclosed in Japanese Patent Kokai No. Sho 63(1988)-117040 and Japanese Patent Kokai No. Hei 7(1995)-157672. Although these particles have novel shapes as resin particles, the shape is plate-like and accordingly those particles do not have remarkable characteristics in comparison with known plate-like inorganic compounds such as talc, mica, etc. And since they are prepared by specific method, it is difficult to produce and use them in an industrial level.
Hence, development of resin particles having high level of functions required in each field of application and suitable for various purposes has been desired.
The present invention was made for meeting such desire, and its subject is to provide resin particles having high level of functions and novel shapes suitable for various purposes and to provide preparation methods thereof.
The inventors studied intensively to solve the problems as mentioned in the above and found that resin particles having novel shapes formed by two curved surfaces or by one curved surface and one plane surface, between which there is a boundary line, can be obtained by mixing a polymeric vinyl monomer and a hydrophobic liquid compound, which has a specific viscosity and does not co-polymerize with the polymerizable vinyl monomer, in a specified ratio and then subjecting the mixture to suspension polymerization in the absence of any crosslinking agent. The inventors further found that number of particles per unit weight of the resin particles thus obtained is larger than those of resin particles having a shape of hemisphere, rugby ball, wooden bowl, go stone as reported in the past, and accordingly the resin particles of the present invention can sufficiently exhibit various effects with smaller amount of content than those of the above known resin particles when they are added to a product for external use.
Thus, according to the present invention, resin particles each of which is formed by two curved surfaces or one curved surface and one plane surface and a boundary line between these surfaces, and satisfies the following numerical relationships:
0.1 xcexcmxe2x89xa6Dxe2x89xa6500 xcexcmxe2x80x83xe2x80x83(I) 
0.05xe2x89xa6d/Dxe2x89xa60.8xe2x80x83xe2x80x83(II) 
wherein D is a particle diameter in horizontal direction and d is the maximum height in longitudinal direction when the boundary line is placed in horizontal direction.
Further, according to the present invention, a method for preparing the resin particles is provided, which is characterized by mixing 100 parts by weight of a polymerizable vinyl monomer and 5-200 parts by weight of a hydrophobic liquid compound which has viscosity of 10-1000000 cSt and does not co-polymerize with the polymerizable vinyl monomer and then subjecting the mixture to suspension polymerization in the absence of any crosslinking agent.
And further, according to the present invention, a method for preparing the resin particles is also provided, which is characterized by mixing 100 parts by weight of a polymerizable vinyl monomer and 41-2000 parts by weight of a fluorinated liquid compound which has viscosity of 0.1-9 cSt at 25xc2x0 C. and does not co-polymerize with the polymerizable vinyl monomer and subjecting the mixture to suspension polymerization in the absence of any crosslinking agent.